Many of us are unaware that every ABS modulator is equipped with multiple filters. These filters can be placed in the inlets, isolation and dump valves and pump or motor. The filters are designed to keep the ABS unit free of debris. Below you can see a few examples of these filters. Most filters are a very fine screen mesh while others are a more course metal mesh. These filters are unserviceable.
Because these filters are unserviceable it is very important to keep them clean. Dirty filters can lead to dirty particles entering the ABS unit. Particles can clog ports and keep actuators from properly sealing. You can inadvertently send sludge and dirty particles up into the ABS unit by depressing a caliper piston too quickly.
This is a brake master cylinder made from acrylic. You can see that it is clean and looks to be capable of performing its proper function.
This is the same acrylic master cylinder with the addition of sludge filled brake fluid. When the caliper piston is depressed too fast it stirs up sludge and quickly sends it back up through the brake system. This sludge can clog ports and malfunction the ABS unit.
There is a simple solution to help prevent sending sludge up into the ABS unit. First, if you have a line lock tool and are able to open the bleed screw then use it to block the brake hose. Then, while depressing the caliper piston, open the bleed screw to allow the brake fluid to escape. Your second option is to not open the bleed screw and don’t block the brake hose. This must be done carefully, but if you take at least 60 seconds to slowly depress the caliper piston then you normally will not stir up any sludge or debris. This will help to keep the ABS filters, ports and actuators clean.
Tech tip: When changing brake pads, take at least 60 seconds to slowly depress a caliper piston.